1. Field of the Invention
The present invention relates to a semiconductor optical functional device, and particularly to an optical functional device which is used for an optical fiber communication system and is capable of performing mode conversion, mode filtering, or optical amplification with respect to a TE mode and a TM mode which are polarization modes in which light orthogonally intersects.
2. Description of the Related Art
Up to now, a semiconductor laser has been often used as a light source in a ultra high speed and long distance optical fiber communication system.
However, in a conventional semiconductor laser, there is a fear that a noise occurs in the inside of the semiconductor laser due to minute reflected returning light from an optical fiber so that a transmission code error occurs.
Thus, in an optical fiber communication system using a semiconductor laser, an optical isolator is generally employed to remove the light returning to the semiconductor laser.
In the above described conventional communication system, the optical isolator is used to remove the returning light to the semiconductor laser. However, the optical isolator requires a number of steps for processing and assembling materials, and is not only expensive but also poor in reliability and stability. Thus, when the optical isolator is used for an optical fiber communication system, there arises a problem that the reliability and stability of the whole system are damaged.